The Venomous Tentacula juice is juice extracted from a Venomous Tentacula plant. It is probably Venomous Tentacula's sap, and acts like a poison. This "poison" is not deadly but, when consumed creates the sensation of burning on the insides. Also, the skin of the person who consumes it becomes a darker tone, usually purple.

Derwent Shimpling probably consumed a large amount of this substance, and his skin colour has become permanently purple.